The present invention relates to direct-current current converters. In particular, the invention concerns a chopper-type direct-current converter, a chopper-type per-type regulator and methods for forming these.
Today, power source constructions aim at a minimum size and a maximum power density, in other words, the aim is to maximize the power to volume ratio of the power source. An obstacle to the reduction of the size of power sources is the physical size of the components and the problem of heat production. The physical size of electronic components, especially semiconductors, is continuously diminishing. The largest components in a power source are magnetic components, such as transformers and filter coils. The size of the magnetic components can be reduced to a certain limit by increasing the switching frequency of the semiconductors. However, an obstacle to increasing the frequency are the losses occurring in the core material of magnetic components and in semiconductors as well as the generation of heat in power sources. If the size of the power source is reduced while the power dissipation remains the same, the surface area emitting heat to the environment is usually also reduced, resulting in a rise in the temperature of the power source. The heat produced is detrimental to other components as well when the power source is placed close to other electronics.
By increasing the switching frequency, it is possible to reduce the size of the magnetic components, but a high frequency results in further losses. It is not sensible to increase the frequency beyond a few hundred kilohertz. A space saving has also been achieved by replacing traditional tall wire-wound magnetic components with low planar structures.
Increasing the switching frequency from the present level to a level sufficient to allow a reduction of the size of magnetic components is not a viable solution in view of the overall effects. The effects produced in the windings by parasitic elements, such as e.g. the losses due to hysteresis of the core material and switching losses, are increased. The work performed by magnetic hysteresis cannot be restored into electric energy; instead, it is converted in the core material into losses, which again increase the temperature of the core material.
Hysteresis losses are increased as the frequency and the alternating component of the magnetic flux are increased. Hysteresis losses account for a substantial portion of the total power dissipation occurring in a magnetic component, such as a transformer or coil. Coil structures get saturated above a certain load current. The advantage achieved by reducing the size of magnetic components is not in proportion to the additional costs arising from the increased power losses.
One method for reducing the size of magnetic components is to integrate several magnetic components around the same magnetic core. Specification U.S. Pat. No. 5,555,494 presents a structure in which several magnetic components have been integrated around the same magnetic core. In the construction, an E-type magnetic core is utilized in which each side leg has an air gap while the center leg is of a continuous structure. Integrated around the magnetic core are the transformer windings and filter coils used for filtering the output voltage of the converter. In this solution, the filter coils are placed around the side legs of the magnetic core.
The object of the present invention is to eliminate the problems referred to above. A specific object of the invention is to disclose a new type of direct-current converter and a chopper-type regulator in which the transformer windings and the output voltage filtering coils are integrated on the same magnetic core. A further object of the invention is to disclose methods for forming a converter and a regulator as specified above.
The invention concerns a method for forming a chopper-type direct-current converter. In this case, the direct-current converter is transformer coupled, so the current supply to the converter is isolated. In other words, there is no galvanic connection between the primary and secondary sides of the converter. The magnetic core of the converter comprises a first and a second side leg, the ends of which are connected to each other by end pieces, and a center leg provided with an air gap and connected to the end pieces between the first and the second side legs. The magnetic core is preferably an E-type structure. Disposed around the magnetic core are a primary winding, a secondary winding and a filter coil for the secondary side. In the method of the invention, the filter coil is placed around the center leg. The primary and secondary windings are so arranged around the side legs that the magnetic flux produced by them flows in the same direction with the magnetic flux of the filter coil.
In a preferred embodiment of the invention, four windings are provided on the primary side of the converter, connecting two windings in series around the first and second side legs. The windings are so arranged around the side legs that the magnetic flux generated by the windings flows in the same direction on each side leg. Further, on the secondary side of the converter, two windings are arranged around the first and second side legs so that the magnetic flux produced by the windings flows in the opposite direction relative to the primary winding placed on the same side leg. The direction of the magnetic flux is the same on each side leg when the magnetic flux can be thought of as circulating around the magnetic core, the path of the flux consisting of the side legs and the end pieces. Thus, the magnetic flux in the first side leg intensifies the magnetic flux in the second side leg.
In an embodiment, the primary windings are controlled by means of a first and a second switching element. Moreover, two capacitors are provided on the primary side, the first capacitor being connected between the switching elements and the second capacitor in parallel with the supply voltage.
In another embodiment, two switching elements and two capacitors are provided on the primary side of the converter, the first switching element being connected between two primary windings and the second switching element correspondingly in series between the other two primary windings. In addition, the first capacitor is connected from the first side of the first switching element to the second side of the second switching element, and the second capacitor is connected from the second side of the first switching element to the first side of the second switching element. The two sides of the switching element may be understood in different ways depending on the switching element; for example, in a MOSFET transistor, the drain is the first side and the source is the second side; similarly, e.g. in a bipolar transistor, the emitter may be the first side and the collector the second side. This definition may be made in accordance with the switching element in question, in a manner known to the person skilled in the art.
In an embodiment, four windings are provided on the primary side of the converter, two windings being connected in series around the first and the second side legs in such manner that the magnetic flux produced by the windings flows in the same direction on each side leg, the other two windings being so connected that the direction of the magnetic flux produced by them on the same side leg is opposite to the flux of the first two windings. Further, two switching elements and a capacitor are provided on the primary side, the first switching element being connected by one end in series with two primary windings and by the other end to the second pole of the supply voltage. The second switching element is connected in a corresponding manner with the other two primary windings. The capacitor is connected in parallel with the supply voltage.
In an embodiment, two switching elements, two capacitors and two windings are provided on the primary side in such manner that the switching elements and the capacitors form a half bridge circuit. The windings are so connected that the magnetic flux produced by the windings flows in the same direction on each side leg and the windings are connected by one end between the switching elements and by the other end between the capacitors.
In an embodiment, four switching elements, a capacitor and two windings are provided on the primary side in such manner that the switching elements form a full bridge circuit. Moreover, the capacitor is connected in parallel with the supply voltage. The windings are connected in series so that the magnetic flux 35 produced by the windings flows in the same direction on each side leg and the windings are connected by one end between two of the switching elements and by the other end between the other two switching elements.
The above-described embodiments may be combined with various embodiments of the secondary side. In one embodiment, the first end of the filter coil winding is connected between the windings on the first and the second side legs and its second end is connected to the first pole of the output voltage of the converter.
In an embodiment, a third and a fourth switching element are provided on the secondary side, connected in series with the secondary winding, and the second pole of the output voltage of the converter is disposed between the third and the fourth switching elements. It is also possible to replace the switching elements with diodes. In this case, the secondary side is provided with a first and a second diode connected in series with the secondary winding, and the second its pole of the output voltage of the converter is disposed between the first and the second diodes.
In an embodiment, at least two different voltage outputs are provided on the secondary side, by providing two windings around the first and the second side legs for each voltage output. The voltage outputs may either be floating or they may have a common ground.
The invention also concerns a method for forming a chopper-type regulator, comprising a magnetic core as described above, with two windings and a filter coil placed around it. The regulator differs from the converter in that a galvanic connection exists between the input and output voltages. In the method, a filter coil is disposed around the center leg and the windings are placed around the side legs so that the magnetic flux produced by them flows in the same direction with the magnetic flux of the filter coil.
The invention further concerns a chopper-type direct-current converter which comprises a magnetic core, a secondary winding and a secondary side filter coil, as described above. In the converter of the invention, the filter coil is wound around the center leg and the primary and secondary windings are wound around the side legs so that the magnetic flux generated by them flows in the same direction with the magnetic flux of the filter coil.
Moreover, the invention concerns a chopper-type regulator comprising a magnetic core as described above, two windings and a filter coil. According to the invention, the filter coil is arranged around the center leg and the windings are arranged around the side legs so that the magnetic flux generated by them flows in the same direction with the magnetic flux of the filter coil.
The advantages of the invention include the fact that the power source can be designed around a single standard type magnetic core. This allows considerable advantages to be achieved both in design and in manufacture. The solution presented allows more effective utilization of the capacity of the magnetic flux density. Reducing the number of separate filter coils decreases the size of the power source and therefore improves its power density. At the same time, the magnetic core can be relatively effectively utilized. In large production quantities, significant cost savings in core material are achieved.
As compared with the traditional transformer solution, the flux variation on the center leg is diminished, resulting in smaller hysteresis and eddycurrent losses in the magnetic core.
Thanks to the structure of the input stage, the drawbacks resulting from the capacitance between large primary windings typical of the push-pull topology can be eliminated, thus eliminating the large current peaks at turn-on of the power semiconductors. This feature reduces the need for filtering in current measurement and it also reduces the current load of the semiconductors. As the decoupling capacitor between the primary switches is charged while the switches are in the non-conduction state, this produces a continuous primary current with a very low ripple. No separate input filter coil is needed, and the electromagnetic interference (EMI) of the input is very small.
In the continuous operating range, the energy stored in the center leg air gap produces a continuous current at the output regardless of the position of the primary switches. This energy is partly discharged via the side leg windings and partly via the center leg winding. Therefore, no separate filter coil is needed at the output. In addition, the side leg secondary windings conducting in push-pull mode allow the use of full wave rectification in which the current load can be distributed equally between two components.
As compared with buck and flyback type power source solutions, the magnetic material can be effectively utilized because magnetization on the side legs of the core occurs in different directions depending on the switching cycle. For the same reason, full wave rectification and other full wave converter principles can be utilized in the analysis.
When considering the operation of the component, we can see that the magnetizing inductance of one transformer is in series with the other transformer. Therefore, in respect of its properties, the component is in a manner a current-fed transformer. Because of this property, current mode control is very well applicable in the control of the component. The series connection of the magnetizing inductance provides an advantage especially in cases of output short circuit and other failure situations, preventing uncontrolled increase of the current.
With the reduction in the operating voltages of electronic circuits, it has become necessary to develop various synchronous rectification methods. The topology described in the present application is also well applicable for use in conjunction with synchronous rectification.